A209049 Smallest pentagonal number with n prime factors (counted with multiplicity).

1, 5, 22, 12, 210, 176, 1520, 1080, 8400, 55200, 273280, 43776, 2624832, 7173360, 71660160, 100659200, 34255872, 178962432, 3623854080, 17895751680, 5413478400, 43690752000, 927712542720, 733008101376, 1789570252800, 35917382287360, 50649120571392

Offset

0,2

Comments

Pentagonal analogy of A075088.

Links

Table of n, a(n) for n=0..26.

Mathematica

k = 1; t = Table[0, {50}]; While[k < 500000001, a = PrimeOmega[k] + PrimeOmega[3 k - 1] - 1; If[ t[[a + 1]] == 0, t[[a + 1]] = k; Print[{k, a}]]; k++]; # (3 # - 1)/2 & /@ t
Join[{1}, Table[SelectFirst[{#, PrimeOmega[#]}&/@PolygonalNumber[5, Range[ 110000]], #[[2]]==n&], {n, 20}][[All, 1]]] (* The program generates the first 21 terms of the sequence. *) (* Harvey P. Dale, Dec 27 2022 *)

Crossrefs

Cf. A075088, A209048.

Sequence in context: A247937 A343840 A270406 * A217444 A063619 A229801

Adjacent sequences: A209046 A209047 A209048 * A209050 A209051 A209052

Keyword

nonn

Author

Robert G. Wilson v, Mar 04 2012

Status

approved